1. Field of the Invention
The present invention relates to ferroelectric capacitors and ferroelectric memory. More particularly, the invention relates to an alloy composition for the bottom electrode of a ferroelectric capacitor, which can be used in a ferroelectric memory.
2. Background Information
Ferroelectric random access memory (FRAM) is a non-volatile memory that maintains stored data without the need for maintaining power to the device. A memory cell (also referred to as a storage node) of an FRAM commonly employs a ferroelectric capacitor having a bottom electrode, a top electrode and a ferroelectrode layer disposed therebetween. The ferroelectric layer is made of a ferroelectric material that exhibits spontaneous electric polarization that can be maintained in the absence of power and that can be reversed in direction by the application of an appropriate electric field. Thus, the polarization direction of the ferroelectric layer of a given memory cell corresponds to a data storage state.
FIG. 1 illustrates a conventional ferroelectric capacitor comprising an iridium (Ir) bottom electrode 102, a lead-zirconate-titanate (PZT) ferroelectric layer 104 and a top electrode 106. U.S. Patent Application Publication No. 2003/0112649 discloses a ferroelectric memory with a ferroelectric capacitor wherein the bottom electrode is an Ir layer or a ruthenium (Ru) layer. U.S. Pat. No. 6,368,910 discloses ferroelectric memories with ferroelectric capacitors wherein the bottom electrode of the capacitor is a platinum (Pt) layer, an Ir layer or a Ru layer. The entire contents of U.S. 2003/0112649 and U.S. Pat. No. 6,368,910 are incorporated herein by reference.
The present inventors have observed, however, that the use of a Pt layer, a Ir layer or a Ru layer as a bottom electrode results in a rough surface of a PZT ferroelectric layer deposited on the bottom electrode, a rougher surface of the PZT layer being indicative of relatively poorer film quality compared to smoother PZT surfaces. Such surface roughness is especially observed when the PZT ferroelectric layer is deposited by metal organic chemical vapor deposition (MOCVD) on Ir, Ru or Pt electrode layers. The relatively poorer film quality of PZT layers that have rougher surfaces can lead to decreased performance of a ferroelectric capacitor in the form of greater leakage current, for example, which can result in greater power consumption and/or poorer retention of stored information in a corresponding FRAM. Thus, greater thicknesses of the ferroelectric layer may be needed to achieve acceptable performance in such devices. In addition, in instances where it is desirable for the bottom electrode layer to comprise a conductive oxide, deposition of iridium oxide is difficult. Moreover, using Ir or Pt for the entire bottom electrode is costly due to the high cost of these metals.